Hydrocarbon bearing reservoirs are complicated, interconnected subterranean systems containing multiple fluid phases. Understanding the connectivity between injection and production wells is a factor in efficient reservoir management as unexpected early water breakthrough at a given well can drastically reduce the oil production rate. Tracer studies provide a means to understand how water is being allocated in the subsurface and to inform the reservoir engineer how to adjust injection rates to mitigate water production. Traditionally, tracer studies are performed by injecting water soluble molecules such as fluorinated benzoic acids and fluorescent dyes such as rhodamine or fluorescein, followed by identification at the producing well via mass spectrometry or fluorescence, respectively. This information may be used to build a map of the fluid pathways in the subsurface environment.
Reservoir simulations of inter-well tracer diffusion over time have demonstrated that tracers injected at different wells may be extracted from a single producer, highlighting the importance of clear and unambiguous tracer signals to aid in distinction. This poses various problems for standard fluorescence-based tracers, which generate overlapping signals given their wide bandwidths of emission (50 nm to 100 nm). This problem, compounded with the limited range of fluorescence detection (300 nm to 1200 nm), the background fluorescence of crude petroleum, and the decline of quantum yield and detectability at higher wavelengths, reduces the amount of distinguishable tracers even for small reservoirs with a moderate number of wells. Moreover, since fluorescence is sensitive to the local environment, salinity, temperature, and the presence of dissolved organic matter make quantitation difficult.
There are other drawbacks to using molecular tracers in the oilfield as well. Due to their small size, molecules tend to diffuse to a greater extent within the matrix as compared to larger entities such as particles, polymers and dendrimers. This leads to lower concentrations at the producing well and greater difficulty in detection. Molecular tracers have to be isolated from the aqueous producing fluid because water is not compatible with gas chromatograph-mass spectrometry (GCMS) instrumentation. This is time consuming and expensive. Each unique molecular tracer has to be vetted for reservoir applications by verifying that the proposed tracer does not stick to the reservoir matrix, is thermally stable and uniquely identifiable. Satisfying all of these specifications drastically reduces the number of potential tracers that could be used. Thus, there is a need to develop an alternative platform to molecular tracers that permits the development of a rich barcoding scheme for elucidating connectivities in complicated, interconnected subterranean systems containing multiple fluid phases.